The compression section of a gas turbine engine has a stator structure and a rotor structure. The rotor structure is disposed about an axis of rotation which extends axially through the compression section. The stator structure is spaced radially from the rotor structure and circumscribes the rotor structure. A flow path for working medium gases extends through the engine and is bounded by the stator structure. Flow directing surfaces are disposed in the flow path to compress the gases in stages (rotor structure) and to direct the gases between compression stages (stator structure).
One example of flow directing surfaces in the rotor structure are the surfaces on each array of rotor blades. The rotor blades extend outwardly from the rotor structure and do work on the incoming gases to compress the gases. Arrays of stator vanes extend inwardly from the stator structure into proximity with the rotor structure. The stator vanes each have a flow directing surface to orient the gases after the gases pass through each stage of rotor blades.
The rotor blades extend into close proximity with the stator structure to minimize leakage around the tips of the rotor blades. Abradable seals are disposed about the interior of the stator structure in close proximity to the rotor structure. These seals block the working medium gases from leak paths around the flow directing surfaces. The stator structure has, for example, an outer abradable seal which extends circumferentially about the tips of the rotor blades to provide a seal at the blade tips. The arrays of stator vanes have an inner abradable seal which extends circumferentially in close proximity to a projecting surface on the rotor assembly, such as a knife edge seal element, to provide a seal at the vane tips. In both cases, it is expected that the rotor assembly as it rotates will cut sealing grooves in the abradable material to minimize any gap under operative conditions. After operation of the gas turbine for a period of time, the abradable material is replaced to repair the seal and insure efficient operation of the engine.
One example of an abradable seal is formed by bonding a layer of porous abradable material, such as fiber metal, having interconnected pores to an arcuate support ring. The support ring is commonly referred to as the substrate. The fiber metal consists of randomly interlocked metal fibers in the form of a sintered and pressed sheet or a sintered and rolled sheet. One satisfactory fiber metal sheet is FELTMETAL.RTM. fiber metal available from the Technetic Division of the Brunswick Corporation, DeLand, Fla. One typical bonding material is braze material such as AMS 4777 (Aerospace Material Specification) braze material. Another abradable seal material which might be formed as honeycomb filled with a porous, abradable material such as FELTMETAL.RTM. fiber metal.
The first step in replacing the abradable material is to remove only the abradable material or the abradable material and braze material. Two examples of methods for removing abradable material, such as fiber metal or honeycomb filled with fiber metal, are shown in U.S. Patents assigned to the assignee of the present invention. One example is shown in U.S. Pat. No. 5,293,717 issued to Snyder et al. entitled "Method For Removal Of Abradable Material Gas Turbine Engine Air Seals". Snyder suggests using abrasive machining to remove abradable material, such as fiber metal or honeycomb, down to the bond interface between the abradable material and the support ring or substrate (See Col. 4, 11. 22-34). U.S. Pat. No. 5,167,721, issued to McComas et al. entitled "Liquid Jet Removal Of Plasma Spray And Sintered", discloses the use of a water jet under high pressure to remove any coating which has a strength less than that of the substrate by adjusting the pressure of the water nozzle. It is noted that the pressure may be adjusted such that it removes the top coat without bond coat damage, or the top coat and bond coat without substrate damage, allowing reuse of the bond coat and substrate or the substrate respectively.
The method of removal of the abradable material that was employed prior to the subject invention uses abrasive machining to completely remove the abradable material and the braze before brazing new fiber metal or, any material attached to the substrate such as honeycomb filled with fiber metal. This causes an occasionally significant loss of parent material in the substrate of 10-15 mils for an abradable seal having an approximate 30 inch diameter with a thickness of fiber metal of 25 mils. Although the machined surface provides an excellent bonding surface because the abradable material and braze have been removed, it requires additional machining time and decreases part life because parent metal is being removed from the surface.
In some applications, only a portion of unfilled honeycomb has been removed before brazing new, untilled honeycomb to the remaining honeycomb. However, all porous, abradable material has been consistently removed, in part, because of the weakness of the porous structure. Thus, both U.S. Pat. No. 5,293,717 and U.S. Pat. No. 5,167,721 suggest methods of removing all of the abradable material or all of the abradable material and braze. Both of these methods work well but may require extensive machining to remove the material or may risk injury to the parent material because of unexpected variations in the curvature of the parts given the small tolerances connected with removing all of the applied braze. For example, the thickness of the braze layer may be as little as eight (8) mils in the construction described above having an abradable material with a thickness of only twenty-five (25) mils on a radius of about thirty (30) inches.
Accordingly, scientists and engineers working under the direction of Applicants' assignee have sought to develop a repair process for replacing porous, abradable seal material which reduces the amount of machining and minimally impacts the service life of the part by reducing the loss of substrate material.